Industrial heating apparatus with airpollution control

ABSTRACT

A stream of hot gas and air, exiting from a drying, baking, or curing oven, and containing pollutants, is diverted through an incinerator where the pollutants are converted to harmless water and carbon dioxide vapors. Some of the hot, purified gas effluent from the incinerator is then directed back to the oven, via an arrangement of dampers, at a rate of flow sufficient to maintain the oven at a predetermined temperature. The dampers exhaust excess hot gas to atmosphere and draw in an equivalent volume of fresh, makeup air. Temperature in the oven is maintained by controlled power means which automatically moves the dampers to vary the proportion of cool, makeup air and hot incinerator effluent entering the oven.

limited ta tea a [m mnn 3,472,498 10/1969 Price etal ABSTRACT: A streamof hot gas and air, exiting from a drying, baking, or curing oven. andcontaining pollutants, is diverted through an incinerator where thepollutants are converted to harmless water and carbon dioxide vapors.Some of the hot, purified gas effluent from the incinerator is thendirected back to the oven, via an arrangement of dampers, at a rate offlow sufficient to maintain the oven at a predetermined temperature. Thedampers exhaust excess hot gas to atmosphere and draw in an equivalentvolume of fresh, makeup air. Temperature in the oven is maintained bycontrolled power means which automatically moves the dampers to vary theproportion of cool, makeup air and hot incinerator effluent entering theoven.

PATENTEU UEBI 4 I97! SHEET 1 BF 3 O O O O O O O O O O l I ll 0 NMO O ,I

/NVEN 70/? gig gem-Zr: Fr '06 grieve Wat/ m a t PATENTEnnacmsn I 352729SHEET 2 BF 3 FYQZ SHEET 3 BF 3 PATENTED DEB 1 419m INDUSTRIAL HEATINGAPPARATUS WITH AIR- POULU'IION CONTROL BACKGROUND OF THE INVENTION Thisinvention relates to industrial, drying, baking and curing ovens, andparticularly to such an oven combined with an incinerator whichsubstantially completely eliminates the discharge of pollutants into theatmosphere.

Industrial pollution from enamel and paint ovens, metalcoating ovens,coffee roasters, core baking ovens, printing presses, brake'liningovens, solvent degreasers, automobile body undercoating ovens, varnishkettles, smoke houses, fat rendering cookers, oil and wax extractors,etc., are polluters of the atmosphere to an even greater extent thanautomobiles, in many industrial areas.

Organic solvents vaporized in these drying, baking and curing operationscontain hydrocarbons, alcohols, aldehydes, esters, ethers, ketones andaromatic compounds as well as fumes containing particulate matter in theform of smoke and carbonaceous material.

When subjected to warmth and sunlight, many of these pollutants createsmog which stings the eyes and irritates the lungs. They are hazardousto health, even deadly in some concentrations, as well as disagreeableand uncomfortable.

The volume of air passed through (and polluted by) these industrialovens is enormous. As one example, a paint drying oven will take between8,000 and 12,000 cubic feet of air (at standard conditions) for eachgallon of paint solvent driven off. In large operations where productslike automobiles, farm implements, or household applicances, aremanufactured in quantities, and painted and dried at the rate they aremanufactured, the volume of air passing through the drying oven amountsto several million cubic feet per day.

The usual way to rid the air of pollutants is to incinerate it bypassing it through high-temperature flames, most commonly fed by naturalgas. It is thoroughly mixed with the flames for a sufficient residenceor dwell time to convert the contaminants to harmless water and carbondioxide vapors.

This is expensive. The capital cost for the initial installation and thecost of the gas are high.

As a consequence of the difficulty of handling large quantities ofpolluted air, and the cost, many of these industrial processes simplydischarge the hot gases into the atmosphere.

SUMMARY OF THE INVENTION A principal object of the present invention isto provide an industrial oven for drying, baking and curing operationswhich will convert polluting components into harmless airborneingredients which may be discharged harmlessly into the atmosphere, andat a reasonable cost.

An object of the present invention is to divert hot gas from an oven toan incinerator, purify it, recycle a portion with cool makeup airthrough the oven depending on the heat requirements of the oven, andvent the rest ofthe purified effluent.

An important feature of the invention is that hot gas from the ovenoutlet may be recirculated for reuse several times, thereby generatingeconomies in reusing heated air and in avoiding the use of a large,expensive incinerator or heater.

Another important object of the invention is that the relatively cold,makeup air is mixed with the hot incinerator effluent outside of theoven, thereby providing efficient, uniform heating throughout the oven,with no cold spots in the oven itself, which would result if makeup airwere admitted directly to the oven.

Another very important feature of this invention is that the sole heatsource may be the incinerator burner, thereby providing economies inusing a single burner for two purposes.

A specific feature of the present invention is the provision of a noveland effective four-way damper means between the incinerator outlet portand the oven inlet port which automatically mixes incinerator effluentwith fresh makeup air to regulate the oven temperature while dischargingan amount of incinerator effluent proportional to the volume of makeupair.

Other objects and advantages will be apparent from the fol lowingdescription taken in connection with the appended drawings in which:

FIG. 1 is a schematic elevational view, in longitudinal cross section,showing an industrial oven with pollution control constructed inaccordance with the present invention and utilizing two blowers forrecycling effective proportions of the oven and incinerator outputsthrough the oven;

FIG. 2 is a cross-sectional plan view of FIG. I taken along the line 22;

FIG. 3 is a fragmentary, enlarged view of FIG. I showing the dampersystem in a nonnal operating position;

FIG. 4 is a view similar to FIG. 3 showing still another operativecondition of the dampers;

FIG. 5 is also similar to FIG. 3, showing still another operativecondition of the dampers; and

FIG. 6 is a view similar to FIG. I of another embodiment of theinvention, utilizing a single blower for recycling through the oven andventing the incinerator effluent.

Like parts are referred to by like reference numerals throughout thefigures.

The preferred embodiments .will now be described, beginning with theembodiment of FIGS. I-5.

A heater housing generally designated 10, includes an oven compartment12 and an incinerator compartment M. As the description proceeds, itwill be apparent that these compartments may be in one integral, unitaryhousing, or for reasons of portability or otherwise, may be made in twoseparate housings for assembly at the job site.

The oven I2 has insulated top, bottom, side and back walls 116, I0, 20,22 and 24. A door 26 provides access to the oven interior. The oven maybe of any suitable size ranging from small, laboratory models to fullsize walk-in or drive-through tunnel types which may be 100 feet long orlonger.

The incinerator M has insulated top, bottom, front, rear and side walls26, 30, 32, 3A and 36.

The walls may be made of any suitable material. One wall constructionwhich has been highly satisfactory is a double wall of spaced steelsheets 30, A0 separated by 3 to 6 inches of glass block insulation,"Fiberglass," rock wool, or a combination thereof, this insulation beingdesignated 42 in FIG. 2.

The oven 112 includes a heating compartment Ml within which articles tobe heated may be stacked, and a recirculation passage 46 separated by avertical wall 43.

A main blower 50 is carried on a shaft 52 which passes through backwall24 and is journaled within bearings 54, 5A fastened to the backwallalong with the motor 56. The motor drives the blower through an endlessbelt 50 and sheaves 60, 62. The blower is surrounded by a shroud 6A toguide air and gas from the blower inlet 66, through the blower. As shownin FIG. 2, air then travels to the left and to the right throughpassages 70, 72 into diffusion ducts 74, 76 comprising vertical plates78 with apertures 80 spaced respectively from the walls 20 and 22.

Air diffusing through the openings 80 travels upward into a duct 82defined by a horizontal plate: 03 spaced downward from the top wall 16.Air enters through openings 0A and passes to the oven compartment outletI36.

At this point, the air/gas mixture comprising the oven atmosphere, nowcontaining pollutants in the form of vaporized hydrocarbon solvents andthe like, is split into two portions. A major portion, preferably 80percent or more, is drawn downward into the main blower inlet 66 forrecirculation through the oven compartment A l.

A minor portion, 20 percent or less. of the air/gas oven atmospherestream at the oven outlet port 06 is diverted by the small, auxiliaryblower 08, through a recycle conduit including ducts and 1104. Duct 90interconnects the oven outlet port 66 and the incinerator inlet port 92.Duct 1104 interconnects the incinerator outlet port 1106 and therecirculation conduit 46 on the intake side of blower 50.

To provide the required dwell or residence time within the incineratorfor proper conversion of the pollutants, the incinerator is made with avertical wall 94. This lengthens the path of travel of the gases so theyare retained within the incinerator, at high reactive temperaturespreferably for V2 to I second.

Immediately upon entering the incinerator, the gases pass downwardbetween the walls 94 and 34, through a gas burner 96. The gas burner andits controls will not be described in detail. Generally, however, theburner will preferably have a large number of closely adjacent apertureand profile plates to reach temperature quickly with a short flamecontinuously across the burner area. The gas entering from the oven willusually have sufficient oxygen (above percent) that additional primaryoxygen will not have to be supplied to the burner 96, but this may bedone (by means not shown), if desired. Secondary oxygen may be supplied,as for example, through an opening diagrammatically illustrated at 98.

For best results, the burner should quickly bring the gas up to atemperature of I,350 to l,500 F. As the hot, mixed gases pass downthrough compartment 100 and up through compartment 102, the pollutantswill be substantially completely oxidized to water and carbon dioxidevapors.

Attention is now directed to the novel, simplified four-way damper means101. This is shown generally in FIG. 1 and in more detail in FIGS. 3, 4and 5, and includes dampers 118 and 122 which will now be described.

As best shown in FIGS. 1 and 4, recycle conduit duct 104 is part of acasing 105 and connects the incinerator outlet port 106 (in wall 32) toan opening 108 in oven wall 24 which leads to the main blower inlet port66 (FIG. 1). A cross-conduit 110 also part of casing 105 connectsconduit 104 to a makeup air inlet port 112. Another cross-conduit 114connects the conduit 104 to an exhaust outlet port 1 16.

A damper 118 is pivoted at the point 120 and is movable between asubstantially wide open position as shown in FIG. 4, and a substantiallyclosed position as shown in FIG. 5. It will seldom be fully closed dueto the necessity of maintaining at least a small flow of fresh airthrough the system to maintain proper combustion and purging ofpollutants.

Another damper 122 is pivoted at the point 124 and is movable between aposition as shown in FIG. 4, where it opens the makeup air port 112 wideand diverts incinerator efi'luent fully upward to the exhaust port 116;and another position as shown in FIG. 5, where it opens the chamber 107and conduit 104 wide all the way from the incinerator to the oven.Coincidentally, it almost closes off makeup air conduit 110. Makeup airshould seldom be completely shut off for the same reason mentioned inconnection with the exhaust.

The two dampers are interconnected for simultaneously opening the makeupair conduit 110 and exhaust conduit 114, and simultaneously closingthem. This is accomplished by levers 126 and 128 connected,respectively, to the dampers 118 and 122. These levers, and theirassociated dampers, are interconnected for simultaneous movement, in thesame direction, by connecting link 130 pivoted at its ends 132 and 134.

The system, as described so far, can function by suitably manuallyoperating the dampers. If the oven becomes too hot, it can be movedsimply by moving the link 130 in an upward direction, from its FIG. 5position, to allow more fresh, cool air to enter the oven. Thesimultaneous opening of dampers 122 and 118 automatically allows theventing of a volume of incinerator effluent proportional to the volumeof fresh makeup air brought in.

Conversely, if the oven becomes too cool, the temperature can beincreased simply by recycling a higher proportion of hot gas from theincinerator. This is accomplished by moving the link 130 downward fromthe FIG. 4 position.

In normal operation, where the oven is operating at a predeterminedtemperature under equilibrium conditions, the dampers will assume someintermediate position such as that shown in FIG. 3, somewhere betweenthe conditions of FIGS. 4 and 5. In FIG. 3, incinerator effluent will besplit, part going out the exhaust conduit 114 and part going past thedamper 122, to mix with fresh makeup air entering from the conduit 1 10.

The novel arrangement of dampers provides a very efficient way ofautomatically maintaining the temperature at any set condition in theoven. This will now be described.

The oven 44 has a servocontrol which maintains the oven at a selectedtemperature. This control may be conventional so is not disclosed herein detail. Briefly, the control includes a thermostat 136 having atemperature-sensing element 138 extending through the top wall 16 intothe heating chamber 44. The thermostat rotates an input shaft 137 of aservo generator 139 one way or another in response to temperaturechanges in the oven. A servomotor 144 operates the damper means 101 toautomatically direct more or less hot incinerator effluent into the ovento maintain it at a temperature selected by the knob 135. The servogenerator and servomotor are connected in parallel to input power linesL, and L usually 1 15 volts 60 cycles alternating current, and areinterconnected by control lines 140, 141, and 142.

The servomotor 144 has a shaft 146 which turns a crank 148 connected bya pivoted link 150 to a crank arm 152 comprising an angular extension ofarm 128.

In operation, counterclockwise rotation of shaft 146 moves the dampersfrom the position of FIG. 4 to the position of FIG. 5; and vice versa.

The automatic temperature control will now be described briefly. Assumethe knob 135 of thermostat 136 is set for an oven temperature of, say350' F. If the oven temperature drops slightly, thereby requiring moreheat to bring the temperature up to the thermostat setting, a signalinitiated by the thermostat 136 will pass through control lines 140,141, 142, calling for counterclockwise rotation of the motor shaft 146.When this occurs, the proportion of cool makeup air through conduit willbe reduced and the proportion of hot gas from the incinerator will becorrespondingly increased. This will bring the oven back up totemperature. Conversely, if, with a setting of 350 F., the temperaturerises slightly, the thermostat 136 willsignal the motor 144, causing thelatter to turn slightly clockwise, thereby increasing the proportion ofcool makeup air and reducing the proportion of hot gas from theincinerator.

As shown in FIG. 1, the auxiliary blower 88 is considerably smaller thanthe main blower 50 for exhausting some of the heated, fumeandvapor-bearing air from the oven. By thus continuously withdrawing someair from the interior of the oven, fumes and vapor are continuouslyexhausted as air is recirculated within the oven. The exhaust blower 88and associated ducts leading to and through the incinerator, have anairflow capacity materially less than the recirculation blower 50, soless air is withdrawn than is recirculated through the oven. Thewithdrawn air, together with fumes and vapors carried by it to theincinerator is replaced by fresh air taken from the exterior through themakeup conduit 110, past the damper 122.

Referring now to the embodiment shown in FIG. 6, this is substantiallythe same as FIG. 1 except that the main blower 50 and recirculationpassage 46 have been eliminated.

A single diffusion duct 174 is located in the floor of the ovencompartment 44 and gases from the damper means 101 pass downward througha duct 176 and through oven inlet port 178 into the plenum space beneaththe duct 174. Thence it passes upward through apertures 179 into oven44. This is, in effect, a simplified version of the FIG. 1 arrangement,in which the single blower 88 furnishes the air and gas circulatingpower required.

The blower 88 directs the entire oven output through first recycle duct90, the incinerator 14, and second recycle duct 104.

The hot, purified effluent from the incinerator in FIG. 6 is dischargedto the four-way damper means 101. As described in connection with FIGS.15, the dampers 118, 122 will operate automatically to proportion thevolumes of hot purified incinerator effluent and cool ambient air whichare directed to the oven inlet port 178 and diffusion duct 1174, inaccordance with the temperature requirements of the oven.Simultaneously, the damper means exhausts a volume of pure, incineratorefi'luent proportioned to the volume of makeup air.

Changes and modification may be made within the scope and spirit of thefollowing claims which define what is believed to be new, in accordancewith the present invention.

lclaim:

1. Apparatus for heating objects to drive off combustible waste productsincluding hydrocarbons, and then to convert the waste products tonon-air-polluting components, the apparatus including:

an oven and an incinerator each having an inlet port and an outlet port;

a recirculation conduit interconnecting the oven inlet and outlet ports,the recirculation conduit having a main blower effective to move a majorportion of the oven atmosphere from the oven outlet port to the oveninlet port for recirculation through the oven;

a recycle conduit interconnecting the oven outlet and incinerator inletports and interconnecting the incinerator outlet and oven inlet ports,said recycle conduit having an auxiliary blower effective to move ovenatmosphere from the oven outlet port via the incinerator to therecirculation conduit on the intake side of the main blower;

the incinerator having burner means for purifying the oven atmosphere byburning the waste products in the oven atmosphere, the incineratorcomprising the major source of heat for oven;

the auxiliary blower being efiective to divert to the incinerator aminor portion of the atmosphere moved from the oven; and

damper means effective to simultaneously draw in ambient makeup air at aselected flow rate, mix the cool ambient air with hot incineratoreffluent in a predetermined proportion and direct the mixture to theintake side of the main blower, and vent incinerator effluent at a flowrate corresponding to said selected flow rate at which ambient makeupair is drawn in.

2. Apparatus according to claim ll, in which the damper means is afour-way damper means.

3. Apparatus according to claim 1, in which the damper means varies theflow of ambient makeup air in said mixture while correspondingly varyingthe flow of hot purified incinerator effluent vented from said recycleconduit, and inversely varying the flow of purified incinerator effluentin said mixture.

4. Apparatus for heating objects to drive off combustible waste productsincluding hydrocarbons, and then to convert the waste products tonon-air-polluting components, the apparatus including:

an oven and an incinerator, each having an inlet port and an outletport;

a recirculation conduit interconnecting the oven inlet and outlet ports;

the recirculation conduit having a main blower effective to move a majorportion of the oven atmosphere from the oven outlet port to the oveninlet port for a repass through the oven;

a recycle conduit comprising first and second recycle ducts;

the first recycle duct connecting the oven outlet port with theincinerator inlet port;

the first recycle duct having an auxiliary blower effective to divert aminor portion of the oven atmosphere to the incinerator;

the incinerator having burner means for purifying the oven atmosphere byburning the waste products in the oven atmosphere and comprising themajor source of heat for the oven;

the second recycle duct connecting the incinerator outlet port with therecirculation conduit on the intake side of the main blower;

the second recycle duct having a casing with a chamber therein andhaving a makeup air inlet port and gas outlet port; and four-way dampermeans in said casing comprising an in- 5 dividual damper for each of theair inlet and gas outlet ports, said dampers being interconnected forsimultaneous movement to vary the inflow of ambient air through the airinlet port while proportionately varying the venting of incineratoreffluent through the gas outlet port and inversely varying the flow ofincinerator effluent to the recirculation conduit. 5. Apparatusaccording to claim 4!, having means for regulating the temperature inthe oven, including a temperaturesensing element in the oven and powermeans for automatically moving the dampers in response to a signal fromthe temperature-sensing element to change the proportion of cool ambientair and hot purified incinerator efi'luent recycled to the oven.

6. Apparatus for heating objects to drive off combustible waste productsincluding hydrocarbons, and then to convert the waste products tonon-air-pollutling components, the apparatus including:

housing means containing an oven and an incinerator, each having aninlet port and an outlet port; a recirculation conduit interconnectingthe oven inlet and outlet ports; a main blower in the recirculationconduit effective to recirculate a major portion of the oven atmospherefrom the oven outlet port to the oven inlet port; a first recycle ductconnecting the oven outlet port with the incinerator inlet port; asecond recycle duct connecting the incinerator outlet port with therecirculation conduit on the intake side of the main blower; anauxiliary blower in the first recycle duct effective to divert theincinerator a minor portion of the oven atmosphere from the oven; theincinerator having burner means for purifying the oven atmosphere byburning the waste products in the oven atmosphere and comprising themajor source of heat for the oven; and four-way damper means in thesecond recycle duct comprising a casing having separate passagesconnected, respectively to: the main blower intake; the incineratoroutlet port; an ambient air makeup inlet port; and an incineratoreffluent exhaust port; a pair of dlampers being interconnected formovement between a. a first position to direct the flow of makeup airfrom the ambient air makeup inlet port to the main blower intake, whiledirecting the flow of incinerator effluent to the incinerator effluentexhaust port, and

b. a second position to direct substantially maximum flow of incineratoreffluent from the incinerator outlet port to the main blower intake,while substantially closing the ambient air makeup inlet port and theincinerator effluent exhaust port.

7. Apparatus according to claim 6 in which movement of the dampersbetween their said first and second positions varies the inflow ofmakeup air through the makeup inlet port in direct proportion tovariation of outflow of incinerator effluent through the exhaust port,and in inverse proportion to variation of flow of incinerator effluentto the recirculation conduit.

d. Apparatus for heating objects to drive off combustible waste productsincluding hydrocarbons, and then to convert the waste products tonon-airpolluting components, the apparatus including:

an oven and an incinerator each having an inlet port and an outlet port;

a recycle conduit interconnecting the oven outlet and incinerator inletports and interconnecting the incinerator outlet and oven inlet ports,said recycle conduit having a blower effective to move oven atmospherefrom the oven outlet port to the oven inlet port via the incinerator;

the incinerator having burner means for purifying the oven atmosphere byburning the waste products in the oven atmosphere, the incineratorcomprising the major source of heat for the oven; and

damper means located in the recycle conduit between the incineratoroutlet port and the oven inlet port effective to simultaneously draw inambient makeup air at a selected flow rate, mix the cool ambient airwith hot incinerator efiluent in a predetennined proportion and directthe mixture to the oven inlet port, and vent incinerator effluent at aflow rate corresponding to said selected flow rate at which ambientmakeup air is drawn in.

9. Apparatus according to claim 8, in which said damper means isfour-way damper means.

10. Apparatus according to claim 8, in which said damper means includesa casing having a chamber connected, respectively, to the incineratoroutlet port and the oven inlet port, an air inlet port, a gas outletport, and individual dampers for the air inlet and gas outlet ports,said dampers being interconnected for simultaneous movement in adirection to correspondingly vary the volume of makeup air entering theair inlet port and incinerator effluent vented through the gas OUI. letport while inversely varying the volume of incinerator effluent recycledto the oven.

11. Apparatus according to claim 10, including a temperature-sensingelement in the oven, power actuated means for moving the dampers, andcontrol means for the power actuated means to move the dampers in adirection to change the proportion of hot incinerator effluent and coolambient air in the mixture directed to the oven inlet port in responseto change of temperature of the sensing element.

12. Apparatus according to claim 11, in which the power actuated meansis effective to increase the proportion of hot incinerator effluent inthe mixture directed to the oven inlet port in response to a drop intemperature below a predetermined value sensed by said element, and isfurther etfective to increase the proportion of cool ambient air in themixture directed to the oven inlet port in response to an increase intemperature above a predetermined value sensed by said element.

2. Apparatus according to claim 1, in which the damper means is afour-way damper means.
 3. Apparatus according to claim 1, in which thedamper means varies the flow of ambient makeup air in said mixture whilecorrespondingly varying the flow of hot purified incinerator effluentvented from said recycle conduit, and inversely varying the flow ofpurified incinerator effluent in said mixture.
 4. Apparatus for heatiNgobjects to drive off combustible waste products including hydrocarbons,and then to convert the waste products to non-air-polluting components,the apparatus including: an oven and an incinerator, each having aninlet port and an outlet port; a recirculation conduit interconnectingthe oven inlet and outlet ports; the recirculation conduit having a mainblower effective to move a major portion of the oven atmosphere from theoven outlet port to the oven inlet port for a repass through the oven; arecycle conduit comprising first and second recycle ducts; the firstrecycle duct connecting the oven outlet port with the incinerator inletport; the first recycle duct having an auxiliary blower effective todivert a minor portion of the oven atmosphere to the incinerator; theincinerator having burner means for purifying the oven atmosphere byburning the waste products in the oven atmosphere and comprising themajor source of heat for the oven; the second recycle duct connectingthe incinerator outlet port with the recirculation conduit on the intakeside of the main blower; the second recycle duct having a casing with achamber therein and having a makeup air inlet port and gas outlet port;and four-way damper means in said casing comprising an individual damperfor each of the air inlet and gas outlet ports, said dampers beinginterconnected for simultaneous movement to vary the inflow of ambientair through the air inlet port while proportionately varying the ventingof incinerator effluent through the gas outlet port and inverselyvarying the flow of incinerator effluent to the recirculation conduit.5. Apparatus according to claim 4, having means for regulating thetemperature in the oven, including a temperature-sensing element in theoven and power means for automatically moving the dampers in response toa signal from the temperature-sensing element to change the proportionof cool ambient air and hot purified incinerator effluent recycled tothe oven.
 6. Apparatus for heating objects to drive off combustiblewaste products including hydrocarbons, and then to convert the wasteproducts to non-air-polluting components, the apparatus including:housing means containing an oven and an incinerator, each having aninlet port and an outlet port; a recirculation conduit interconnectingthe oven inlet and outlet ports; a main blower in the recirculationconduit effective to recirculate a major portion of the oven atmospherefrom the oven outlet port to the oven inlet port; a first recycle ductconnecting the oven outlet port with the incinerator inlet port; asecond recycle duct connecting the incinerator outlet port with therecirculation conduit on the intake side of the main blower; anauxiliary blower in the first recycle duct effective to divert theincinerator a minor portion of the oven atmosphere from the oven; theincinerator having burner means for purifying the oven atmosphere byburning the waste products in the oven atmosphere and comprising themajor source of heat for the oven; and four-way damper means in thesecond recycle duct comprising a casing having separate passagesconnected, respectively to: the main blower intake; the incineratoroutlet port; an ambient air makeup inlet port; and an incineratoreffluent exhaust port; a pair of dampers being interconnected formovement between a. a first position to direct the flow of makeup airfrom the ambient air makeup inlet port to the main blower intake, whiledirecting the flow of incinerator effluent to the incinerator effluentexhaust port, and b. a second position to direct substantially maximumflow of incinerator effluent from the incinerator outlet port to themain blower intake, while substantially closing the ambient air makeupinlet port and the incinerator effluent exhaust port.
 7. Apparatusaccording to claim 6 in which movement of the dampers between their saidfirst and second positioNs varies the inflow of makeup air through themakeup inlet port in direct proportion to variation of outflow ofincinerator effluent through the exhaust port, and in inverse proportionto variation of flow of incinerator effluent to the recirculationconduit.
 8. Apparatus for heating objects to drive off combustible wasteproducts including hydrocarbons, and then to convert the waste productsto non-air-polluting components, the apparatus including: an oven and anincinerator each having an inlet port and an outlet port; a recycleconduit interconnecting the oven outlet and incinerator inlet ports andinterconnecting the incinerator outlet and oven inlet ports, saidrecycle conduit having a blower effective to move oven atmosphere fromthe oven outlet port to the oven inlet port via the incinerator; theincinerator having burner means for purifying the oven atmosphere byburning the waste products in the oven atmosphere, the incineratorcomprising the major source of heat for the oven; and damper meanslocated in the recycle conduit between the incinerator outlet port andthe oven inlet port effective to simultaneously draw in ambient makeupair at a selected flow rate, mix the cool ambient air with hotincinerator effluent in a predetermined proportion and direct themixture to the oven inlet port, and vent incinerator effluent at a flowrate corresponding to said selected flow rate at which ambient makeupair is drawn in.
 9. Apparatus according to claim 8, in which said dampermeans is four-way damper means.
 10. Apparatus according to claim 8, inwhich said damper means includes a casing having a chamber connected,respectively, to the incinerator outlet port and the oven inlet port, anair inlet port, a gas outlet port, and individual dampers for the airinlet and gas outlet ports, said dampers being interconnected forsimultaneous movement in a direction to correspondingly vary the volumeof makeup air entering the air inlet port and incinerator effluentvented through the gas outlet port while inversely varying the volume ofincinerator effluent recycled to the oven.
 11. Apparatus according toclaim 10, including a temperature-sensing element in the oven, poweractuated means for moving the dampers, and control means for the poweractuated means to move the dampers in a direction to change theproportion of hot incinerator effluent and cool ambient air in themixture directed to the oven inlet port in response to change oftemperature of the sensing element.
 12. Apparatus according to claim 11,in which the power actuated means is effective to increase theproportion of hot incinerator effluent in the mixture directed to theoven inlet port in response to a drop in temperature below apredetermined value sensed by said element, and is further effective toincrease the proportion of cool ambient air in the mixture directed tothe oven inlet port in response to an increase in temperature above apredetermined value sensed by said element.